Wireless communication networks, such as cellular networks, operate by sharing resources among mobile terminals operating in the communication network. As part of the sharing process, resources are allocated by one or more controlling devices within the system. Certain types of wireless communication networks are used to support cell-based high speed services such as those under the family of IEEE 802.16 standards. The IEEE 802.16 standards are often referred to as WiMAX or less commonly as WirelessMAN or the Air interface Standard. Another emerging standard that has not yet been ratified is referred to as Long Term Evolution (LTE). Other wireless networking technologies include Third Generation (3G), Third Generation Partnership Project (3GPP), and 802.11, popularly known as WiFi.
More specifically, IEEE 802.16e extends the 2004 version of IEEE 802.16 for fixed Broadband Wireless Access so as to support the mobility of users and provide Quality of Service (QoS) guarantees to enable multimedia services. From a system-level perspective, an IEEE 802.16e cell includes a number of Mobile Stations (MSs) served by a Base Station (BS), which controls the access to the wireless medium in a centralized manner. Before transmitting to (or receiving from) the BS, a MS must request the admission of a new connection. If accepted, the BS is then responsible for meeting the requested QoS guarantees.
The shared wireless medium demands co-ordinated transmission of multiple traffic flows over it. Duplexing refers to the way two-way communication is carried on the transmission medium. There are two commonly used duplexing techniques: Time Division Duplex (TDD), and Frequency Division Duplex (FDD). In TDD, the DL and UL traffic is typically transmitted on the same carrier frequency at different times. The time allocations for the DL and UL portions can be adaptive, which makes it suitable for asymmetric connections. In FDD, the UL and DL traffic is transmitted on different carrier frequencies, and may thus be transmitted/received simultaneously. An FDD hybrid known as Half-duplex Frequency Division Duplex (H-FDD), adds the restriction that a terminal cannot transmit and receive at the same time. H-FDD is cheaper to implement and less complex than full-duplex FDD, though the system throughput is lower.
Access to the shared wireless medium is scheduled using Orthogonal Frequency-Division Multiple Access (OFDMA) frames that extend over two dimensions: time, in units of OFDMA symbols, and frequency, in units of logical subchannels. Data bursts are conveyed into two-dimensional (i.e. time and frequency) data regions, which identify regions within the frame and are advertised by the BS via specific control messages. Each frame is divided into downlink (DL) and uplink (UL) subframes. The former is used by the BS to transmit data to the MSs, whereas the MSs transmit to the BS in the latter.
FIG. 12 shows an example TDD frame structure. As shown, a DL subframe starts with a preamble followed by a Frame Control Header (FCH), a downlink MAP (DL-MAP), and an uplink MAP (UL-MAP). The preamble helps MSs perform synchronization and channel estimation. The FCH specifies a burst profile and the length of one or more downlink bursts that immediately follow the FCH in the current frame. The DL-MAP and UL-MAP notify MSs of the corresponding resources allocated to them in the downlink and uplink direction, respectively, within the current frame. In general, the BS is free to define the shape and position of any data region. Based upon the schedule received from the BS, each MS can determine when (i.e., OFDMA symbols) and where (i.e., subchannels) it should receive from and transmit to the BS. Proper time gaps, namely receive-to-transmit transition gap (RTG) and transmit-to-receive transition gap (TTG, also referred to herein as TRG), have to be inserted between consecutive subframes in order to give wireless devices sufficient time to switch from transmission mode to reception mode, or vice versa.
FIG. 13 shows an example FDD frame structure. IEEE 802.16 specifies that BSs of FDD systems shall operate in full-duplex mode, while MSs shall be either full-duplex (FDD) or half-duplex (H-FDD). The FDD frame structure shown in FIG. 13 supports the concurrent operation of H-FDD and FDD MSs. The frame structure supports a coordinated transmission arrangement of two groups of H-FDD MSs (Group-1 and Group-2) that share the frame at distinct partitions of the frame. As shown, the DL frame contains two subframes. DL Subframe 1 comprises a preamble region, a MAP region (MAP1) and a data region (DL1). DL Subframe 2 comprises a MAP region (MAP2) and a data region (DL2). Similarly, the UL frame contains two subframes, UL2 and UL1. FIG. 13 shows the timing relationship of the UL subframes relative to the DL subframes. The four parameters TTG1, TTG2, RTG1 and RTG2 are sufficiently large to accommodate the H-FDD MSs transmit receive switching time plus the round trip propagation delay. Group-1 H-FDD MSs listen to DL Subframe 1 and transmit in uplink subframe UL1. Group-2 H-FDD MSs listen to DL Subframe 2 and transmit in uplink subframe UL2. The MAP regions—MAP1 and MAP2—are independent and include FCH, DL-MAP and UL-MAP.
IEEE 802.16j adds multihop relay capabilities to IEEE 802.16 systems. Relay-based systems typically comprise low-cost relays, which are associated with specific base stations (BSs). The relays can be used to extend the coverage area of a BS and/or increase the capacity of a wireless access system. The relays can repeat transmissions to/from the BS so that MSs within communication range of a relay can communicate with the BS through the relay. The relays do not need a backhaul link because they communicate wirelessly with both BSs and MSs. This type of network may be referred to as a multihop network because there may be more than one wireless connection between the MS and a hardwired connection. Depending upon the particular network configuration, a particular MS may gain network access via one or more neighbour relays and/or one or more neighbour BSs. In addition, relays themselves might have one or more available path options to connect to a particular BS. IEEE 802.16j requires that from the perspective of the MS any communications with a Multihop Relay Base Station (MR-BS) which are relayed through a Relay Station (RS) appear to be the same as if they had come directly from the BS. The radio link between a MR-BS or RS and an MS is called an access link, while the link between a MR-BS and an RS or between a pair of RSs is called a relay link.
IEEE 802.16j defines two different RS modes of operation: transparent and non-transparent. A Transparent RS (T-RS) does not transmit control information such as preamble, FCH, and MAP. An MS connected to a T-RS receives control information directly from the MR-BS, and the T-RS relays only data traffic. A Non-Transparent RS (NT-RS) transmits a preamble and other broadcast messages and relays data traffic as well.
IEEE 802.16j specifies a TDD frame that is divided into DL and UL subframes, much like the IEEE 802.16 TDD frame structure shown in FIG. 12. However, IEEE 802.16j subframes are further divided into zones to support BS-RS communications and RS-MS communications in addition to BS-MS communications. In both transparent and non-transparent modes, so-called “access zones” are defined that support BS/NT-RS communications with the MS/T-RS. In the transparent mode a so-called “transparent zone” is defined for T-RS communications with the MS. In non-transparent mode “relay zones” are defined for BS/NT-RS communications with NT-RS. FIG. 14 shows an example configuration for a T-RS frame structure. FIG. 15 shows an example configuration for a T-RS frame structure in which MR-BS and RS have partitioned the UL subframe in the frequency domain. FIG. 16 shows an example of a minimum configuration for a NT-RS frame structure. FIG. 17 shows an example of configuration for NT-RS frame structure where MR-BS and RS have partitioned the UL subframe in the frequency domain.
Presently, the available standards for relay operation such as the IEEE 802.16j standard support only a TDD frame structure, and therefore, only the TDD mode of operation. However, systems such as WiMax, IEEE 802.16e and LTE support FDD, H-FDD and TDD capable mobile terminals.
A need exists for FDD and H-FDD support in multihop relay networks such that the coverage and other performance enhancements of relay systems can be extended to the FDD-based systems.